London: British Prime Minister David Cameron said on Tuesday that police in Britain may get temporary powers to seize the passports of British nationals fighting in the Middle East who are attempting to return to the United Kingdom to conduct terrorist operations.

Mr Cameron said in a statement to the House of Commons that his government was looking at "specific and discretionary powers" that would effectively prevent those suspected of having terrorism-related motives from returning to Britain.

Under the proposals, militants returning home from places such as Iraq and Syria would have their passports seized at the border. Official government figures put the number of British Islamic State fighters operating in Syria and Iraq at up to 800.

British Prime Minister David Cameron is mulling changes in the way the UK deals with dual citizens who go abroad to fight for terrorist organisations. Photo: AP

Mr Cameron said that more needed to be done not only to stop Islamic extremists from returning to Britain but also to address extremists already in the country.

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He said passports were not an automatic right and could be taken away.

"This power will include appropriate safeguards and oversight arrangements," he said.

He said it was "abhorrent" that British nationals were opting to fight for extremist groups such as the Islamic State.

However, it is not yet clear whether a more aggressive proposal to strip returning British nationals of their citizenship – not the same as forcing them to renounce passports – will be accepted by legislators, with the Prime Minister's political partners, the Liberal Democrats, suggesting such a move may be illegal.

Menzies Campbell, the former leader of the Liberal Democrats, expressed legal concerns over who would decide when to introduce a ban over particular individuals. He also questioned whether there would be a right to appeal.

Mr Cameron said the initiative to strip suspected terrorists of their citizenship would not apply to British nationals who held one passport and thereby would be stateless as a result.

The proposed action would apply to British nationals who held two passports.

"We can deprive dual nationals of their citizenship to stop them returning. We can bar foreign nationals on the basis of the threat they pose," he said.

He said new laws would be drawn up to reflect current aviation security arrangements.

"Airlines will have to comply with our no-fly list arrangements, give us information on passenger lists and comply with our security screening requirements. If they do not do this, their flights will not be able to land in Britain," Mr Cameron said.

Late last week, Britain raised its terror threat level from "substantial" to "severe", meaning a terrorist attack in Britain was considered highly likely.

However, the government has said there is no evidence to suggest an attack is imminent.

Separately, British police confirmed as a hoax on Tuesday a message that circulated on social media at the weekend that the London Underground tube network would be the subject of an imminent attack. The message briefly raised traveller anxiety.

"Adhering to British values is not an option or a choice," Mr Cameron said in his statement, his first to Parliament since July. "It is a duty for all those who live in these islands so we will stand up for our values. We will in the end defeat this extremism, and we will secure our way of life for generations to come."